This invention relates to a cathode ray tube for use in a television receiver, a display for a personal computer, and the like.
Referring to FIG. 1, a cathode ray tube comprises as an envelope a glass bulb which includes a glass panel 11 with a fluorescent film 10 formed on an inner surface thereof, a flare-shaped funnel 12 formed behind the glass panel 11, and a neck 13 with an electron gun arranged therein, as well known in the prior art. In the inner surface of the glass panel, a shadow mask 14 is arranged to face the fluorescent film 10. An electron beam emitted from the electron gun is irradiated through the shadow mask 14 onto the fluorescent film 10 so that an image is displayed on a front portion (hereinafter referred to as a face portion) of the glass panel 11.
The interior of the above-mentioned cathode ray tube is kept in a high vacuum state so that the glass bulb is subjected to compressive stress and tensile stress. If mechanical shock is applied to the glass bulb, there is a risk of occurrence of implosion. In view of the above, the face portion of the glass panel generally has a thickness greater in its peripheral region than in its central region, for the purpose of maintaining the sufficient strength of the cathode ray tube.
Accordingly, the typical glass panel is liable to have a difference in light transmittance between a central region and a peripheral region of the face portion. As a result, the luminance in the peripheral region is reduced in comparison with the central region so that the image becomes nonuniform in brightness and is therefore hard to watch.
In addition, various kinds of glasses are used for the glass panel in dependence upon applications thereof. As the transmittance of the glass itself forming the glass panel is higher, the difference in transmittance resulting from the difference in thickness between the central region and the peripheral region of the face portion becomes smaller but the contrast is decreased.
From the above-mentioned background, a proposal is made of a cathode ray tube which has a reduced difference in transmittance between the central region and the peripheral region thereof by using a glass panel made of a glass having a high transmittance, and which has an improved contrast by bonding a colored glass plate to a face portion with an adhesive.
In recent years, the glass panel is required to have a flatness. Accordingly, it is tried to increase the radius of curvature of the face portion in a diagonal direction of the outer surface thereof to 10000 mm or more. However, the greater the radius of curvature of the face portion of the glass panel is, the higher the risk of the implosion becomes. Therefore, in case where the radius of curvature of the face portion of the glass panel in the diagonal direction of the outer surface thereof is equal to 10000 mm or more as described above, the face portion is designed so that the thickness (TE) of the peripheral region in the diagonal direction and the thickness (T.sub.0) of the central region satisfy the formula of 1.2.ltoreq.TE/T.sub.0.ltoreq.4.0.
However, when the difference in thickness of the face portion of the glass panel is too great between the central region and the peripheral region as described above, it is difficult to sufficiently reduce the difference in transmittance between the central region and the peripheral region even if the glass panel is made of the glass having a high transmittance.
Furthermore, the greater the thickness of the face portion of the glass panel is, the heavier the cathode ray tube becomes. The use of the colored glass plate adhered to the front surface thereof further increases the weight of the cathode ray tube. As a result, transportation is difficult.
In addition, when the colored glass plate is adhered, bubbles may be mixed in the adhesive or striae may be produced so that the image is difficult to watch. Furthermore, the adhesive may be deteriorated during the use over a long period of time so that the colored glass plate is detached.